The instant invention relates generally to variable displacement axial piston pumps and, more specifically, to a device for changing pump displacement.
A common type of axial piston pump includes a housing having a rotatably mounted barrel with a plurality of circumferentially spaced cylinder bores. A port plate is interposed between the barrel and the inlet and working ports of the device so to alternately connect each cylinder bore with the inlet and working ports as the barrel is rotated. Within each bore is a piston which is connected by shoes to a thrust plate assembly mounted on a pivotal rocker cam assembly. When the barrel is rotated the shoes slide across the thrust plate, which causes the pistons to reciprocate and pump fluid.
In one form of variable displacement axial piston pump, the rocker cam assembly is pivoted about an axis perpendicular to the axis of rotation of the barrel to vary the inclination of the thrust plate assembly. This changes the stroke of the pistons and, consequently, changes the displacement of the pump. An example of such a pump is disclosed in U.S. Pat. No. 3,967,541. In thispatent, a manually operated valve on the pump regulates pressure fluid flow to a fluid motor which, when operated, changes the inclination of the rocker cam.
In some installations, it is not convenient to manually operate a pump mounted control device to set and change the displacement of a pump. For example, in some installations, a pump and its attendant control device are inaccessible to an operator and the control device cannot be operated manually.
It is desirable to provide a control device for a variable displacement pump which can be operated from a remote location. One type of control device which can be operated from a remote location, or which can operate another device located remotely therefrom, is an electrically controlled servo valve. U.S. Pat. No. 3,401,711 discloses a single receiver, electrically operated, jet-type servo valve. The servo valve has an electric force motor which operates a pilot stage and a main stage. The main stage includes a spool valve which controls the flow of fluid to a hydraulic motor or similar device which can be at a remote location. The operation of the spool valve is controlled by the pilot stage.
A problem with using a servo valve where the pilot stage operates a spool valve, which in turn controls fluid flow to a hydraulic device such as a fluid motor to set the displacement of a pump, is that the pump cannot be easily adapted to manual operation which is desirable in the event the control fails. Accordingly, it is desirable to provide an electrically operated control for a pump which also allows the pump to be manually operated.